This invention pertains to superplastic forming of two or more layers of sheet metal. More specifically this invention relates to a method for locating and temporarily bonding a sheet metal reinforcement piece to a larger sheet metal blank so that the sheet metal blank and reinforcing sheet can be deformed together in the same elevated temperature stretch forming operation.
It is well known that some sheet metal alloys can be subjected to thermo-mechanical processing to yield a microstructure permitting very high elongation under tensile stress at elevated temperature. Suitable aluminum, magnesium, stainless steel and titanium alloys can be processed in this way. Sheet metal blanks of these alloys can then be heated to a suitable forming temperature and stretched over a forming tool or into a die cavity to make complex panels and other parts. These materials are said to be superplastic alloys, or superplastically formable alloys (SPF alloys) and the stretch shaping processes are called superplastic forming (SPF) processes.
SPF alloys such as cold rolled AA5083 sheet stock can be used to form automobile deck lid inner and outer panels, lift-gate inner and outer panels and other body and closure panels. The aluminum alloy sheet metal starting material is hot and cold rolled to a very fine grain size of about ten micrometers. Sheet metal blanks of this material are heated to a temperature of about 450xc2x0 C. to 500xc2x0 C. for SPF shaping. They are stretched at relatively high strain rates under the pressure of a suitable working gas such as air, nitrogen or argon against a forming tool or die into a large panel of detailed and complex configuration. In fact, single SPF parts can often replace many separately formed and welded stamped parts of lower elongation material.
AA5083 sheet blanks are often cold rolled to about one to three millimeters in thickness and, as stated, can be SPF stretch formed into strong body or closure panels. However, sometimes it is desirable to increase the thickness of at least some portion of the panel. If the entire panel is to be thicker it is usually possible to form two sheets of like profile at the same time. The sheets are welded or gripped at their edges and deformed together against the forming tool. However, if only small regions of the blank are to be reinforced it is much more difficult to form and apply stiffening or reinforcing pieces to small selected spots. It has proven difficult to suitably shape patches or reinforcing sheet pieces and apply them to the desired shaped spot on an automotive panel.
This invention provides a method of locating and temporarily bonding a sheet metal reinforcing piece, or other useful metal piece, to a sheet metal blank preparatory to a SPF operation on the two layers. In accordance with a preferred embodiment of the invention, the smaller piece(s) is positioned and bonded on the blank sheet to undergo the same deformation as the adjacent blank sheet region that is intended to be reinforced or otherwise benefited by the second object. A suitable adhesive has been discovered that maintains the reinforcement in place during the stretch forming operation so that the reinforcement sheet takes the same shape as the adjacent region of the metal blank. After forming, the reinforcing piece is removed from the formed sheet until it is desired to permanently fix it to a formed sheet metal part.
The practice of the invention is facilitated by using a suitable high temperature adhesive to bond one or more reinforcing pieces, or other useful parts, to the base sheet metal blank. The adhesive must be easy to apply before forming and easy to remove after forming. The adhesive must be capable of surviving the aggressive heating and forming steps. And the adhesive must be chemically compatible with the sheets so as not to corrode or degrade them. In accordance with the invention, an aqueous solution or dispersion of sodium silicate, i.e., water glass, is preferred. More broadly, an aqueous solution (or dispersion) of an alkali silicate, containing one or more of sodium, potassium or lithium, may be used.
For example, superplastic aluminum sheet alloys, such as AA 5083 of a suitably fine grain microstructure, are used in the superplastic stretch forming of automobile body panels. The designer of such a body panel may wish to provide reinforcing strips of the same alloy composition in selected regions of the panel. An aqueous solution of sodium silicate of suitable concentration and viscosity is very useful in sticking small reinforcing pieces of the alloy to the main sheet metal blank before forming. After the forming operation and suitable cooling of the shaped parts, water can be used to soften the silicate bond and permit removal of the shaped reinforcement piece(s) from the main panel piece. Afterwards, the reinforcement piece can be permanently attached to the panel such as by welding, riveting or other desired means.
Other objects and advantages of the invention will become apparent from a detailed description of a preferred embodiment of the invention.